Transcriptional, Metabolic, Physiological and Developmental Responses to Nitrogen Limitation in Switchgrass (Panicum virgatum)

Sustainable switchgrass production (Panicum virgatum) as a bioenergy crop hinges partly on efficiently using soil macronutrients, especially nitrogen (N). In this study, switchgrass plants were grown under four different N conditions (6.0mM KNO3, N-replete control; 2.0mM, mild limitation; 0.6mM, moderate limitation; and 0.2mM, severe limitation). Subsequently, the physiological, metabolic, and transcriptomic responses of switchgrass roots and shoots to N limitation were determined.Moderate N limitation marked a tipping point for large changes in plant growth, root-to-shoot ratio, root system architecture (root surfaces and specific root length - SRL) and total nitrogen content. Integrating metabolic data and transcriptomic analyses revealed that N limitation reduced switchgrass photosynthetic capacity and carbon(C)-fixation activities. Our results indicate that switchgrass balances C-fixation with N-assimilation and N transport and recycles N-compounds by rerouting C-flux from glycolysis, the oxidative branch of the pentose phosphate pathway (OPPP), and from the tricarboxylic acid (TCA) cycle in an organ-specific manner. The energy and reduction power so generated, and C-skeletons appear to be directed towards N uptake, biosynthesis of energy storage compounds with high C/N ratio such as sucrose, non-N-containing lipids, and various branches of secondary metabolism. Taken together, the findings of this study provide new insights into how switchgrass plants adapt to N limitation.